The present invention relates to an apparatus for producing soft X-rays by impinging a high energy laser beam or other high energy electrically charged particle beam on a metal target. More particularly, the invention relates to an apparatus for producing soft X-rays from plasma for use in an X-ray exposure apparatus in a semiconductor manufacturing system.
As now generally known, soft X-rays can be generated by producing plasma at high temperature and high density. Plasma having sufficiently high temperature and high density to generate soft X-rays can be instantly produced by impinging a high energy beam, such as a laser beam, upon the surface of a solid metal target. The soft X-rays generated by plasma have a number of characteristics: (1) the X-rays have high brightness; (2) an X-ray beam is produced with a neary pin point source; (3) the X-rays are characterized by a short pulse; and (4) if an YAG-laser which has a large duty cycle is used, the X-ray source can be used for many repetitions.
Plasma X-ray apparatus have been used experimentally in submicron X-ray lithography and various measurement apparatus. In such experimental apparatus, a laser beam has been used to impinge on a target formed by a solid metal surface placed inside a vacuum vessel. Such experimental apparatus is disclosed in "Sov. Phys. Tech.", Phys., 28(7), p. 863 (1983) and "Submicron X-ray Lithography Using Laser-Produced Plasma As A Source", B. Yaakobi et al., Appl. Phys. Lett. 43(7), I, pgs. 686-688 (October 1983). However, experimental apparatus of the above type cannot be used in a practical application because of several disadvantages. First, regular replacement of the target material would be required, which would in turn necessitate breaking the vacuum of the vessel to remove the old target material and install new target material. This replacement procedure would be inconvenient and cause a considerable loss of time. Secondly, in the above experimental apparatus, windows are included on the vessel through which the laser beam or the X-rays pass. Because the target material is a solid metal, plasmatized material is formed which adheres to the windows of the vessel resulting in a decrease in the effective power of the laser beam impinging on the target material and lessening of the strength of the X-rays produced by the plasma.
An apparatus for preventing adhesion of plasma material on the windows of the vessel of a plasma X-ray apparatus is disclosed in Japanese Patent Disclosure No. 58-158842 filed in the name of S. Iwamastu on Sept. 21, 1983. The X-ray apparatus disclosed in this application uses a refrigerated target material such as a piece of ice, solid ammonia, solid Argon, Krypton or Xenon, or solid carbonic acid (dry ice). Such refrigerated target materials turn to gas upon being subjected to a high energy laser beam which avoids the problem of particles of plasma adhering to the windows of the vessel. However, the above apparatus is greatly complicated by the fact that the target material must be refrigerated. The apparatus also requires a complicated supply mechanism for the target material as discussed in an article in "Japanese Atomic Engineering", 26(7) pg. 594 (1984).